Hydrogen absorbing alloy and nickel-metal hydride rechargeable battery

ABSTRACT

An object of the present invention is to provide a hydrogen absorbing alloy which can improve a high rate discharge property while suppressing particle size reduction, exhibits cycle life characteristics equal to or higher than those of conventional alloys even when its cobalt content is decreased, and has a high capacity. Specifically, the present invention provides a hydrogen absorbing alloy having a CaCu 5  type crystal structure in its principal phase, wherein the La content in the alloy is in the range of 24 to 33% by weight and the Mg or Ca content in the alloy is in the range of 0.1 to 1.0% by weight, as well as the aforesaid alloy wherein the Co content in the alloy is not greater than 9% by weight.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] This invention relates to hydrogen absorbing alloys, and moreparticularly to a hydrogen absorbing alloy which can be used to formnegative electrodes for use in nickel-metal hydride rechargeable(secondary) batteries.

[0003] 2. Description of the Related Art

[0004] Conventionally, misch metal (hereinafter referred to as “Mm”)comprising a mixture of rare earth elements such as La, Ce, Pr, Nd andSm, and nickel-base alloys formed by replacing a part of Ni with variouselements are widely used as hydrogen absorbing alloys for formingnegative electrodes for use in nickel-metal hydride rechargeablebatteries.

[0005] It is known that, among others, cobalt-containing alloys arecapable of absorbing a relatively large amount of hydrogen, are lessliable to particle size reduction in their hydrogen-loaded state, haveexcellent corrosion resistance in alkalis, and are effective inprolonging the lives of nickel-metal hydride rechargeable batteries whenthey are used for the negative electrodes thereof.

[0006] On the other hand, it is also known that lower cobalt contentsare more desirable for an improvement of a high rate discharge property.The reason for this is believed to be that a decrease in cobalt contentpromotes particle size reduction and hence causes an increase in surfacearea per unit weight.

SUMMARY OF THE INVENTION

[0007] In order to solve these problems of the prior art, the presentinvention provides a hydrogen absorbing alloy which can improve a highrate discharge property while suppressing particle size reduction,exhibits cycle life characteristics equal to or higher than those ofconventional alloys even when its cobalt content is decreased, and has ahigh capacity.

[0008] The present invention is based on the discovery that, when ahydrogen absorbing alloy has a relatively high La content and containsan alkaline earth metal (i.e., Mg or Ca) in a relatively small amountabove impurity levels, the alloy can improve a high rate dischargeproperty in spite of suppressed particle size reduction whilemaintaining its high capacity, and can suppress particle size reductioneven when its cobalt content is decreased to less than theconventionally known level.

[0009] Specifically, the present invention relates to a hydrogenabsorbing alloy having a CaCu₅ type crystal structure in its principalphase, wherein the La content in the alloy is in the range of 24 to 33%by weight and the Mg or Ca content in the alloy is in the range of 0.1to 1.0% by weight.

[0010] In a preferred embodiment, the present invention also relates tothe aforesaid alloy wherein the cobalt content in the alloy is notgreater than 9% by weight.

[0011] When-the hydrogen absorbing alloy of the present invention isused for the negative electrode of an alkaline rechargeable battery, itcan increase the capacity of the battery, can improve the high ratedischarge property thereof, and can suppress particle size reductioneven at low cobalt contents to cause a reduction in battery cost.

BRIEF DESCRIPTION OF THE DRAWING

[0012]FIG. 1 is a x-ray diffraction pattern for the hydrogen absorbingalloy of Example 1.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0013] In the AB₅ type hydrogen absorbing alloy of the presentinvention, 0.1 to 1.0% by weight of Mg or Ca is contained in order toimprove the high rate discharge property while suppressing particle sizereduction. Moreover, the La content in the alloy is set at 24 to 33% byweight in order to increase the amount of hydrogen absorbed and controlthe equilibrium pressure of hydrogen. Thus, as contrasted withconventional alloys, the hydrogen absorbing alloy of the presentinvention has a high capacity, can improve the high rate dischargeproperty while suppressing particle size reduction, and can enhanceresistance to particle size reduction even at low cobalt contents.

[0014] Such hydrogen absorbing alloys may, for example, be expressed interms of the following chemical formulas:

La_(u)R_(v)Mg_(w)Ni_(x)Co_(y)M_(z), and

La_(u)R_(v)Ca_(w)Ni_(x)Co_(y)M_(z)

[0015] wherein R is a rare earth element other than La, M is at leastone element chosen from the group consisting of Mn, Al, Si, Sn, Fe, Cu,Ti, Zr, and V or the like, the content of La is preferably 24 to 33% byweight, R 15% by weight, Mg or Ca 0.1 to 1.0% by weight, Ni 50 to 60% byweight, Co 9% by weight or less, and M 3 to 10% by weight. Here, thecompositional ratios of elements are expressed in terms of atomic ratios(u, v, w, x, y and z). These atomic ratios can be obtained by dividingthe percentage-by-weight for each element by the respective atomicweight and then by normalizing the resulting figures using the sum ofconstitutional ratios of La and R, which are classified as “A” elements.Thus, u plus v equals 1 by definition. Because R is a rare earth elementwhich is other than La, and M is at least one element chosen from thegroup of Mn, Al, Si, Sn, Fe, Cu, Ti, Zr, V or the like, the weightedaverages of atomic weights are used for R and M. Excluding Mg and Ca,which are added in minor amounts, as well as unavoidable impurities, theratio of elements belonging to “B” to those belonging to “A” iscalculated as a B/A ratio according to the following equation: B/A ratio=(x+y+z)/(u+v).

[0016] Furthermore, in the AB₅ type hydrogen absorbing alloy of thepresent invention, the remainder of the moiety A comprises one or morerare earth elements other than La, and the remainder of the moiety Bcomprises one or more transition metals such as Ni, Co and Mn and/or Alor the like. The atomic ratio of B to A, B/A, is preferably 4 to 7, morepreferably 5 to 7, further more preferably 5 to 6.

[0017] The AB₅ type hydrogen absorbing alloy used in the presentinvention is preferably a hydrogen absorbing alloy having a CaCu₅ typecrystal structure in its principal phase. As used herein, the expression“hydrogen absorbing alloy having a CaCu₅ type crystal structure in itsprincipal phase” refers to a hydrogen absorbing alloy in which, althoughsegregation phases are partly recognized by metallographic observationof a section, the diffraction pattern recorded by XRD exhibits a CaCu₅type alloy phase.

[0018] The hydrogen absorbing alloy of the present invention ischaracterized in that its Mg or Ca content is in the range of 0.1 to1.0% by weight. If its Mg or Ca content is less than 0.1% by weight, theeffect of suppressing particle size reduction will be insufficient. Ifits Mg or Ca content is greater than 1.0% by weight, the amount ofhydrogen absorbed will be decreased to an undue extent.

[0019] When the Co content is lowered, the equilibrium pressure ofhydrogen at the time of absorption or desorption of hydrogen iselevated. Accordingly, the La content is set at 24 to 33% by weight inorder to maintain the equilibrium pressure of hydrogen at the same levelas those of conventional alloys and to maintain or improve the highcapacity. In the present invention, it is especially preferable to addMg.

[0020] Moreover, the present invention involving the addition of arelatively small amount of Mg or Ca as described above makes it possibleto achieve a long life at cobalt contents of not greater than 9% byweight, preferably 7% by weight or less, and more preferably 6% byweight or less, as contrasted with the prior art in which it has beenunachievable.

[0021] The addition of a small amount of one or more selected from thegroup consisting of Ti, Zr and V to the Mg- or Ca-containing hydrogenabsorption alloy can enhance the initial characteristics or cycle lifecharacteristics. The amount of the addition is as small as 0.5% byweight or less based the Mg- or Ca-containing hydrogen absorbing alloy.

[0022] Moreover, the Mg-containing hydrogen absorbing alloy has a CaCu₅type crystal structure in its principal phase, wherein the length fora-axis (a-axis =b-axis) is in the range of 4.990 to 5.050 Å, the lengthfor c-axis is in the range of 4.030 to 4.070, regarding the latticeconstants thereof. Comparing the lattice constants in these rangebetween Mg-free and Mg-containing hydrogen absorbing alloys, theaddition of Mg tends to increase the lattice constants. It has beenparticularly found that increase for c-axis is larger than that fora-axis so that the ratio of length of c-axis to length of a-axis, c/a,becomes larger.

[0023] It has been found that increase in the ratio c/a results in lessliability to particle size reduction so as to produce a battery withlonger cycle life. The reason for this is believed to be that the largerface distance between the face perpendicular to c-axis, which are thefaces for closest packing of crystal, suppress the extension of thelattice. Consequently, the stress is restrained and the developingdistances for cracks become smaller. Thus, the less liability to theparticle size reduction for the hydrogen absorbing alloy comprising 0.1to 1.0% by weight of Mg is thought to be derived from c-axis havinglonger extension than a-axis.

[0024] Further, it has been fount that, the hydrogen absorbing alloy,having 24 to 33% by weight of La, 6 to 9% by weight of Co and the atomicratio B/A of 5.0 to 5.25, and which Mg is added to in an amount of 0.1to 1.0% by weight, can result in a battery with higher capacity such as340 mAh/g or more, keeping cycle life unchanged. In this case, the ratioB/A means the sum of atomic ratios of, for example, Ni, Co, Mn and Al,excluding the elements in very small amounts such as Mg and Ca, bydesignating the sum of atomic ratios of rare-earth metals such as La,Ce, Pr and Nd for one.

[0025] The hydrogen absorbing alloy of the present invention can bemanufactured by a dissolution method such as arc dissolution and highfrequency dissolution, casting in a mold, table-casting, a rapid rollquenching method, gas atomization, disk-atomization or a spin-cupmethod, or a combination thereof.

[0026] The hydrogen absorbing alloy of the present invention may beprepared in the following manner.

[0027] Predetermined amounts of various elements may be weighed out andmelted in a high-frequency furnace having an atmosphere of an inert gas(at 200 to 1,500 Torr) such as Ar gas. In the case of an element (e.g.,Mg or Ca) having a high vapor pressure, it may be added directly byitself, or in the form of an alloy formed of such an element and one ormore other elements constituting the alloy. In the melting method, itmay be preferable that Mg or Ca is not added until metals with highmelting point such as Ni or Co are melted in order to prevent addedcomponents from evaporating or assure the safe operation. The resultingmelt may be cast in a mold made of iron at a temperature of 1,300 to1,600° C. to form an ingot. Or the other methods mentioned above may bealso used. In case of special need, the ingot may be heat-treated at atemperature of 800 to 1,200° C. for 5 to 20 hours in an inert atmosphere(at 600 to 1,500 Torr), for example, of Ar gas.

[0028] Using a jaw crusher, a roll mill, a hummer mill, a pin mill, aball mill, a jet mill, a roller mill and the like, the hydrogenabsorbing alloy prepared in the above-described manner may be ground toan average particle diameter of 4 to 70 μm in an inert atmosphere, forexample, of Ar. Moreover, reduction in particle size by hydrogenabsorption and desorption, so-called hydrogenation method, may be used.Thus, there can be obtained a hydrogen absorbing alloy in accordancewith the present invention.

[0029] The hydrogen absorbing alloy powder thus obtained may be formedinto electrodes according to any well-known method. This can beaccomplished, for example, by mixing the alloy powder with a binderselected from polyvinyl alcohol, cellulose derivatives (e.g.,methylcellulose), PTFE, polyethylene oxide, high polymer latices and thelike, kneading this mixture into a paste, and applying this paste to anelectrically conducting three-dimensional support (e.g., foamed nickelor fibrous nickel) or an electrically conducting two-dimensional support(e.g., punching metal). The amount of binder used may be in the range of0.1 to 20% by weight per 100% by weight of the alloy.

[0030] Moreover, if necessary, an electrically conducting filler such ascarbon-graphite powder, Ni powder or Cu powder may be added in an amountof 0.1 to 10% by weight based on the alloy.

[0031] Alkaline batteries using the hydrogen absorbing alloy of thepresent invention for the negative electrodes thereof have a long cyclelife and exhibit an excellent high rate discharge property andlow-temperature discharge characteristics, even when the alloy has a lowcobalt content.

[0032] The present invention is further illustrated by the followingexamples. However, these examples are not to be construed to limit thescope of the invention.

EXAMPLE 1 AND COMPARATIVE EXAMPLE 1

[0033] Mm or rare earth elements such as La, Ce, Pr and Nd, metallicelements such as Ni, Co, Mn and Al, and Mg were weighed out so as togive each of the compositions shown in Table 1. Mg was used in the formof a MgNi₂ (m.p. 1100° C.) alloy. These materials were melted in ahigh-frequency melting furnace, and the resulting melt was cast in amold made of iron to form an ingot. As for the Mg-free alloy, the ingotis formed without using the Mg-Ni alloy.

[0034] This ingot was heat-treated at 1,050° C. for 6 hours in anatmosphere of Ar. Thereafter, using a grinder, this ingot was ground toan average particle diameter of 33 μm so as to obtain a hydrogenabsorbing alloy powder. Analysis of this allow powder by XRD revealedthat it had a CaCu₅ type crystal structure (FIG. 1).

[0035] 10 g of this powder was mixed with 2.5 g of a 3 wt % aqueoussolution of polyvinyl alcohol (with an average degree of polymerizationof 2,000 and a degree of saponification of 98 mole %) to prepare apaste. This paste was filled into a porous metallic body of foamednickel in an amount of 30% by volume, dried, and then pressed into aplate having a thickness of 0.5-1.0 mm. Finally, a negative electrodewas made by attaching a lead wire thereto.

[0036] A positive electrode, which comprised a sintered electrode, wasbonded to the aforesaid negative electrode with a polypropyleneseparator interposed therebetween. This assembly was immersed in a 6NKOH electrolyte to construct a battery.

[0037] Each of the batteries so constructed was tested in the followingmanner. First of all, at a temperature of 20° C., the battery wascharged to 120% at 0.3C (90 mA/g) based on the capacity of the negativeelectrode, rested for 30 minutes, and then discharged at 0.2C (60 mA/g)until the battery voltage reached 0.6 V. When this cycle was repeatedtwenty times, the greatest discharge capacity was regarded as the“capacity” of the alloy. Subsequently, this battery was charged to 120%at 0.3C, and discharged at 2.0C (600 mA/g). The capacity measured inthis manner was regarded as the “high rate discharge capacity”.Thereafter, in order to observe the degree of particle size reduction,the negative electrode was disassembled, placed in water, and exposed toultrasonic waves from an ultrasonic horn so as to separate the alloypowder from the current collector. The particle size distribution afterrepeated charging and discharging was measured by means of a Microtrackanalyzer to determine the average particle diameter D₅₀ (μm). Theresults thus obtained are shown in Table 1. It is to be-understood that,when the frequency of occurrence of each particle diameter in themeasured particle size distribution is cumulatively added from thesmaller to the larger side, the particle diameter corresponding to 50%of the entire distribution is defined as D₅₀. TABLE 1 High rate Averageparticle discharge diameter after Alloy composition (wt %) Capacitycapacity size La Ce Pr Nd Mg Ni Co Mn Al (mAh/g) (mAh/g) reduction (μm)Example 1 25.04 3.16 1.90 1.30 0.27 56.61 5.31 4.58 1.82 305 220 25.31Comparative 25.45 3.21 1.94 1.32 0.00 53.76 8.64 3.77 1.92 302 162 23.14Example 1

[0038] As shown in Table 1, Mg-containing alloy has a better high ratedischarge property and less liability to particle size reduction.

EXAMPLES 2-5 AND COMPARATIVE Example 2

[0039] The compositions shown in Table 2 were employed for the formationof alloys in the same manner as Example 1 and capacities were measuredin the same matter as Example 1 to examine the relationship between theLa content and the capacity when magnesium is contained in the alloys.The results thus obtained are shown in Table 2. It can be seen fromTable 2 that, in order to obtain an alloy having a high capacity, the Lacontent in the alloy must be not less than 24% by weight. TABLE 2 Alloycomposition (wt %) Capacity La Ce Pr Nd Mg Ni Co Mn Al (mAh/g) Example 225.56 3.87 1.30 1.33 0.17 58.86 2.71 3.79 2.42 306 Example 3 25.06 3.791.27 1.30 0.16 58.92 2.66 4.46 2.37 297 Example 4 24.86 3.76 1.26 1.290.27 59.22 2.64 3.69 3.02 293 Example 5 24.69 3.74 1.25 1.28 0.27 58.812.62 5.00 2.34 289 Comparative 23.80 6.25 1.32 1.35 0.29 57.44 2.77 4.382.41 275 Example 2

EXAMPLES 6-8 AND COMPARATIVE Example 3

[0040] Employing the compositions shown in Table 3, alloy powders wereprepared in the same manner as in Example 1. Then, electrode tests werecarried out in the same manner as in Example 1 to determine therespective capacities. The results thus obtained are shown in Table 3.It can be seen from Table 3 that Mg contents of greater than 1.0% byweight cause an undue reduction in capacity. TABLE 3 Alloy composition(wt %) Capacity La Ce Pr Nd Mg Ni Co Mn Al (mAh/g) Example 6 26.59 3.871.30 1.33 0.17 58.94 2.71 3.80 2.30 306 Example 7 25.53 3.86 1.29 1.330.28 58.80 2.71 3.79 2.42 301 Example 8 24.97 3.78 1.27 1.30 0.55 58.692.65 4.44 2.36 286 Comparative 24.99 3.15 1.27 1.30 1.09 58.74 2.65 4.452.37 270 Example 3

EXAMPLES 9-12 AND COMPARATIVE EXAMPLES 4-7

[0041] Employing the alloy compositions shown in Table 4, electrodetests were carried out in the same manner as in Example 1. Thereafter,each negative electrode was disassembled, placed in water, and exposedto ultrasonic waves from an ultrasonic horn so as to separate the alloypowder from the current collector. The particle size distribution afterrepeated charging and discharging was measured by means of a Microtrackanalyzer to determine the average particle diameter D₅₀ (μm). On thebasis of the average particle diameter of an alloy containing no Mg, theeffect of Mg addition, i.e. the improvement of particle size reduction,was calculated as R1 (%) according to the following equation.

R1(%)={(D₅₀ (μm) of Mg-containing alloy)/(D₅₀ (μm) of Mg-freealloy)}×100 (%)

[0042] Since the degree of particle size reduction varies greatly withthe Co content, the improvement of particle size reduction isindependently shown with respect to each Co content. The D₅₀ is definedin such a way that, when the particle size distribution of the hydrogenabsorbing alloy is measured and the frequencies of detection of variousparticle diameters are cumulatively added from smaller-diameter tolarger-diameter particles, the particle diameter corresponding 50% ofall particles is represented by D₅₀. TABLE 4 Improvement of particlesize Alloy composition (wt %) reduction La Ce Pr Nd Mg Ni Co Mn Al R1(%) Example 9 25.53 3.86 1.29 1.33 0.28 58.80 2.71 3.79 2.42 129.2Example 10 24.97 3.78 1.27 1.30 0.55 58.69 2.65 4.44 2.36 134.7Comparative 25.60 3.87 1.30 1.33 0.00 58.96 2.72 3.80 2.42 100.0 Example4 Comparative 25.56 3.87 1.30 1.33 0.08 58.86 2.71 3.79 2.42 104.0Example 5 Example 11 25.51 3.86 1.29 1.32 0.28 56.19 5.41 3.78 2.35121.7 Comparative 25.58 3.87 1.30 1.33 0.00 56.34 5.43 3.79 2.36 100.0Example 6 Example 12 25.38 3.84 1.29 1.32 0.28 53.62 8.61 3.76 1.91110.5 Comparative 25.45 3.85 1.29 1.32 0.00 53.77 8.64 3.77 1.92 100.0Example 7 Example 12 25.37 3.84 1.29 1.32 0.28 53.20 9.42 3.39 1.91103.0 Comparative 25.44 3.85 1.29 1.32 0.00 53.09 9.45 3.65 1.92 100.0Example 8

[0043] It can be seen from Table 4 that the addition of Mg suppressesparticle size reduction at the same Co content, and this effect becomesmore pronounced as the Co content is decreased. It can also be seenthat, at a low Mg content, for example, of less than 0.1% by weight, theimprovement of particle size reduction is as low as 5% or less.Moreover, it can also be seen that, at a high Co content, for example,of greater than 9% by weight, the effect of Mg addition is lessened. Incommercially available nickel-metal hydride rechargeable batterieshaving a high capacity, the Co content is usually not less than 9%.However, it can be seen that the present invention exhibits asignificant effect at Co contents of not greater than 7%.

EXAMPLES 14-17 AND COMPARATIVE EXAMPLES 8-11

[0044] Employing the alloy compositions shown in Table 5, the alloypowders were prepared in the same manner as in Example 1 except thefollowing: metallic Mg (m.p. 650° C.) was used instead of the Mg-Nialloy, and the mixture of Ni, Co, Mn, Al and some of rare-earth elementswere melted in advance, and then after confirming the melting, the otherof rare-earth elements and metallic Mg were added. As for the Mg-freealloys, the melting was carried out without addition of the metallic Mg.

[0045] The capacity in Table 5 was measured as follows. After dry-mixinghydrogen absorbing alloy 0.5 and Ni power 1.5 in the weight ratio, themixture was molded in a mold with a diameter of 20 mm to produce anelectrode. The battery was charged to 125% at 0.5C (150 mA/g), restedfor 10 minutes, and then discharged at 0.5C (150 mA/g) until the voltagedifference based on mercury reference electrode (Hg/HgO) reached 0.6v.After this cycle was repeated ten times, the capacity was measured (aspellet capacity).

[0046] Moreover, Cycle life was measured as follows. Using theabove-mentioned sample battery having the paste electrode, at atemperature of 20° C., the battery was charged to 120% at 0.3C (90 mA/g)based on the capacity of the negative electrode, rested for 30 minutes,and then discharged at 0.2C (60 mg/g) until the battery voltage based onthe positive electrode reached 0.8V. This cycle for charge and dischargewas repeated two hundred times, and the maintenance of dischargecapacity (cycle life) was calculated using the next equation.

Maintenance(%) ={(discharge capacity after 200 cycles)/(dischargecapacity after 20 cycles)}×100

[0047] Further, using the above-mentioned sample battery having thepaste electrode, at a temperature of 20° C., the battery was charged at0.3C (90 mA/g) based on the capacity of the negative electrode, restedfor 30 minutes, and then discharged at 0.2C (60 mA/g) until the batteryvoltage reached 0.8V. After this cycle was repeated twenty times, inorder to observe the degree of particle size reduction, the battery wasdisassembled and the alloy powder for the negative electrode was exposedto ultrasonic waves from an ultrasonic horn so as to separate the alloypowder from the current collector. The particle size distribution afterrepeated charging and discharging was measured by means of a Microtrackanalyzer to determine the average particle diameter D₅₀ (μm). Theimprovement of particle size reduction R1 was calculated.

[0048] The diffraction patterns for the alloys shown in Table 5 weremeasured using a X-ray diffraction method for powder. The latticeconstants were calculated based on the measurement data using a methodof least squares. TABLE 5 Improve- ment of particle Capac- size LengthLength ity Cycle reduction of of Elongation Elongation Alloy composition(wt %) (mAh/ life R1 a-axis c-axis of a-axis of c-axis La Ce Pr Nd Mg NiCo Mn Al g) (%) (%) (Å) (Å) (Å) (Å) Example 14 25.07 3.79 1.27 1.30 0.2757.88 3.99 4.46 1.95 330 90 117 5.020 4.063 0.002 0.007 Comparative25.14 3.80 1.28 1.31 0.00 58.04 4.00 4.48 1.95 320 82 100 5.018 4.056Reference Reference Example 8 Example 15 25.04 3.79 1.27 1.30 0.27 56.365.31 4.83 1.82 335 93 114 5.024 4.061 0.001 0.006 Comparative 25.11 3.801.27 1.30 0.00 56.51 5.33 4.84 1.83 330 85 100 5.023 4.055 ReferenceReference Example 9 Example 16 28.07 3.86 0.00 0.00 0.28 55.81 6.77 3.661.55 350 83 115 5.033 4.048 0.000 0.008 Comparative 28.15 3.87 0.00 0.000.00 55.97 6.79 3.67 1.55 330 70 100 5.033 4.040 Reference ReferenceExample 10 Example 17 28.07 3.86 0.00 0.00 0.28 54.59 8.12 3.53 1.55 34890 107 5.034 4.046 0.001 0.006 Comparative 28.15 3.87 0.00 0.00 0.0054.74 8.14 3.54 1.55 325 80 100 5.033 4.040 Reference Reference Example11

[0049] As shown in Table 5, forcusing on the effects of the addition ofMg, the addition of Mg increases capacity, cycle life, and improvementof particle size reduction. Comparison of the lattice constants showsthat the addition of Mg tends to increase the c-axis more remarkablythan a-axis. This is thought to be one of the reasons why the highercapacity and increased cycle life are attained. The results for Examples16 and 17 show the specific increase of discharge capacity, althoughincrease of cycle life therefor is fair.

EXAMPLES 18 TO 32, COMPARATIVE EXAMPLES 12 TO 19

[0050] Employing the alloy compositions shown in Table 6, the alloypowders were prepared using MgNi₂ (m.p. 1100° C.) in the same manner asin Example 1 except the following: the mixture of Ni, Co, Mn, Al andsome of rare-earth elements were melted at first. Then, after theconfirmation of the melting, the other of rare-earth elements and theMg-Ni alloy were added for melting. As for the Mg-free alloys, themelting was carried out without addition of the metallic Mg.

[0051] Pellet capacity and maintenance of discharge capacity (cyclelife) were obtained as the same manner as described above. After theaverage particle diameter D₅₀ was obtained as the same manner asdescribed above, the improvement of particle size reduction wascalculated as R2 (%) according the following equation. The R2 shows theinhibition effect against particle size reduction for the alloys otherthan the alloy for Example 16 on the basis of the average particlediameter of the alloy of Comparative Example 16.

R2(%)={(D₅₀ (μm) of Mg-containing alloy)/(D₅₀ (μm) of the alloy forExample 16 alloy)}×100 (%) TABLE 6 Improvement of Cycle particle sizeAlloy composition (wt %) Ratio Capacity life reduction R2 La Ce Pr Nd MgNi Co Mn Al B/A (mAh/g) (%) (%) Example 18 28.74 3.22 0.00 0.00 0.2853.30 8.81 3.79 1.86 5.20 340 93 137 Example 19 28.74 3.22 0.00 0.000.28 53.98 8.13 3.79 1.86 5.20 350 90 130 Example 20 28.74 3.22 0.000.00 0.28 54.66 7.45 3.79 1.86 5.20 350 88 130 Comparative 28.82 3.230.00 0.00 0.00 52.77 9.51 3.80 1.87 5.20 315 90 130 Example 12Comparative 28.82 3.23 0.00 0.00 0.00 53.45 8.83 3.80 1.87 5.20 320 83130 Example 13 Comparative 28.82 3.23 0.00 0.00 0.00 54.13 8.15 3.801.87 5.20 323 77 125 Example 14 Comparative 28.82 3.23 0.00 0.00 0.0054.81 7.47 3.80 1.87 5.20 328 73 115 Example 15 Comparative 28.82 3.230.00 0.00 0.00 55.49 6.79 3.80 1.87 5.20 330 89 100 Example 16 Example21 28.74 3.22 0.00 0.00 0.28 52.63 9.48 3.79 1.86 5.20 325 97 140Example 22 28.74 3.22 0.00 0.00 0.28 55.33 6.77 3.79 1.86 5.20 350 83120 Example 23 30.34 0.97 0.32 0.33 0.28 53.98 8.13 3.79 1.86 5.20 35088 130 Example 24 28.74 2.58 0.32 0.33 0.28 53.97 8.13 3.79 1.86 5.20350 90 130 Example 25 27.14 3.86 0.65 0.33 0.28 53.97 8.13 3.79 1.865.20 345 92 130 Example 26 23.93 6.44 0.97 0.66 0.28 53.94 8.12 3.791.86 5.20 340 92 130 Comparative 32.03 0.00 0.00 0.00 0.00 54.15 8.153.80 1.87 5.20 350 75 110 Example 17 Comparative 21.13 7.31 1.23 6.270.00 51.03 7.69 3.58 1.76 5.20 330 92 130 Example 18 Comparative 20.799.04 1.30 1.00 0.00 54.07 8.14 3.80 1.86 5.20 320 93 130 Example 19Example 27 27.89 3.84 0.00 0.00 0.28 56.15 6.70 3.62 1.53 5.25 345 85121 Example 28 28.07 3.86 0.00 0.00 0.28 55.86 6.74 3.64 1.54 5.20 35085 118 Example 29 28.26 3.89 0.00 0.00 0.28 55.56 6.79 3.67 1.55 5.15350 83 114 Example 30 28.46 3.91 0.00 0.00 0.28 55.26 6.83 3.69 1.565.10 355 80 110 Example 31 28.65 3.94 0.00 0.00 0.28 54.95 6.88 3.721.57 5.05 355 75 107 Example 32 28.85 3.97 0.00 0.00 0.29 54.64 6.933.75 1.59 5.00 360 70 105

[0052] As shown in Table 6, the alloys keeping La content of 24 to 33%by weight and Co content of 6 to 9% by weight in addition of 0.1 to 1.0%by weight of Mg with the B/A atomic ratio of 5.0 to 5.25, enables theachievement for the higher capacity such as 340 mAh/g or more ofcapacity, although the cycle life therefor is as usual.

1. A hydrogen absorbing alloy having a CaCu₅ type crystal structure inits principal phase, comprising La in the range of 24 to 33% by weightin the alloy, and Mg or Ca in the range of 0.1 to 1.0% by weight in thealloy.
 2. A hydrogen absorbing alloy according to claim 1, furthercomprising 9% by weight or less of Co in the alloy.
 3. A hydrogenabsorbing alloy according to claim 1, further comprising 6% by weight orless of Co in the alloy.
 4. A hydrogen absorbing alloy according toclaim 1, wherein the Co content is 6 to 9% by weight, and the atomicratio B/A is 5.0 to 5.25, where A represents a rare earth elementincluding La, and B represents a rare earth element, transition metal orAl.
 5. A hydrogen absorbing alloy according to claim 1, furthercomprising one or more selected from the group consisting of Ti, Zr andV.
 6. A hydrogen absorbing alloy having a CaCu₅ type crystal structurein its principal phase, comprising Mg and having a-axis length of 4.990to 5.050 Å and c-axis length of 4.030 to 4.070 Å for the latticeconstants in the CaCu₅ type crystal structure.
 7. A hydrogen absorbingalloy according to any one of claims 1 to 4 having a-axis length of4.990 to 5.050 Å and c-axis lenth of 4.030 to 4.070 Å for the latticeconstants in the CaCu₅ type crystal structure.
 8. A method formanufacturing a hydrogen absorbing alloy having a CaCu₅ type crystalstructure in its principal phase, characterized in that a Mg-supplymaterial is added to dissolution of component elements for hydrogenabsorbing alloy in an amount of 0.1 to 1.0% by weight in an entirehydrogen absorbing alloy.
 9. A method for manufacturing a hydrogenabsorbing alloy according to claim 8, characterized in that at least Niand Co are melted in a melting vessel, and then the Mg-supply materialis added to the melting vessel.
 10. A method for manufacturing ahydrogen absorbing alloy according to claim 8 or 9, characterized in theMg-supply material is selected from metallic Mg and Mg alloy withmelting point of 650° C. or higher.
 11. A nickel-metal hydriderechargeable battery using the hydrogen absorbing alloy of any one ofclaims 1 to 7 for an electrode thereof.